SRT default maximum live payload size 1316 -> 1456

[maxsharabayko]

Use SRT_LIVE_MAX_PLSIZE = 1456; // MTU(1500) - UDP.hdr(28) - SRT.hdr(16)
instead of
SRT_LIVE_DEF_PLSIZE = 1316; // = 188*7, recommended for MPEG TS
by default for live transfer mode.

Still payload of 1316 can be fed into SRT and transmitted.

[ethouris]

The SRTO_PAYLOADSIZE option should rather be "partially removed" - that is, it can still be set to some smaller value and may stay there for backward compat, but it will be deprecated and will be only partially regarded. This option should be maintained internally and for measurement it should:

• start from 0
• set EQUAL to the incoming report value when the current value is 0 (this will never happen again during transmission)
• otherwise add a brick for IIR average measurement